The principal objective of the present research plan is to evaluate how humoral and mechanical factors regulate the turnover of total and specific myofibrillar proteins in adult rabbit cardiac myocytes maintained in cell culture. These nonproliferating myocyte preparations will be pulse labeled with tracer amino acids to measure the fractional rates of protein synthesis and degradation at specific intervals after altering insulin, thyroid hormone and norepinephrine levels. Similarly, these quiescent myocytes will be stimulated to beat or periodically stretched to determine how these physical parameters alter protein turnover. Changes in the fractional rate of protein synthesis will also be compared to alterations in total poly A messenger RNA content in order to determine whether message transcription and rate of protein synthesis fluctuate in parallel. The expression of a major myofibrillar protein, myosin heavy chain (MHC), will be investigated by employing a cDNA probe to determine whether the transcription of MHC genes is altered in response to changing culture conditions. The subcellular distribution of myosin will also be monitored by immunofluorescent and immunoelectron microscopy using monoclonal antibodies directed against myosin and, in particular, the MHC alpha. The structural organization of the contractile apparatus will be examined morphometically in an attempt to describe how this complex set of proteins is assembled and broken down under conditions that alter the fractional synthesis and degradation rates of specific myobifrillar proteins. The role of the lysosomal vacuolar apparatus in mediating the degradation of total and specific cardiac proteins will also be pursued. The present experiments are designed to estimate the contribution of hormonal modulation, contractile activity and passive stretch on the synthesis and degradation of myocytic protein. These studies will provide basic information on how cultured cardiac myocytes regulate their protein composition and advance our understanding of the mechanisms that control cardiac mass under differing physiologic and pathophysiologic states.